The proposals aim at the elucidation of the biochemical basis of hypercholesterolemias in man by a collaborative effort between biochemists and clinicians. 1. In Project A work on the elucidation of a metabolic shunt of intermediates of sterol biosynthesis derived from mevalonate but not leading to sterols, and discovered in the P.I.'s laboratory, is proposed by the use of C13 and C14-labelled mevalonates and substances derived from it. Experiments on animals and tissue preparations will be made. Since impairments of the shunt may lead to hypercholesterolemia, a quantitative method, developed by the applicants, for measuring the extent of the shunt in men and women will be applied to normal, hyper- and hypo-cholesterolemic individuals. The method depends on the determination of C14O2 exhaled in 24 hours after administration of a tracer dose of mevalonate. A correlation will be sought between the extent of the shunt and nature of hypercholesterolemias. 2. In Project B the use of human leukocytes and cultured fibroblasts is proposed for studying genetic and epigenetic factors (hormones, drugs, diet, stress) that might control sterol biosynthesis and especially levels of 3-hydroxy-3- methylglutaryl-CoA reductase in adults and children. The applicants discovered that the leukocytes of heterozygous familial hypercholesterolemics responded with an abnormally rapid induction of this enzyme in a lipid- free medium. The abnormality is thought to result from a control gene mutation. Evidence will be sought to determine whether decreased binding of lipoproteins to cell surfaces or a rapid dissociation of an endogenous repressor from cells accounts for the genetic defect. Experiments with labelled lipoproteins, and with the serum and cells of an abetalipoproteinemic individual are also being proposed. A comparison will be made between the behavior of leukocytes and fibroblasts of the same individual, and a correlation will be made between the extent of "the shunt" (cf. above) and the metabolism of isolated cells.